This invention relates generally to absorption refrigeration systems and more particularly to corrosion inhibiting materials for use in such systems and to a method of providing such materials to an absorption system. Corrosion inhibitors are commonly provided to an absorption system to reduce the corrosion of metals in the system that are in contact with the system working fluid.
There are a number of different pairs of refrigerants and absorbents that are used in absorption refrigeration systems. One such pair is water and a halogen salt of lithium, such as lithium bromide, lithium chloride or lithium iodide, mixed to form a concentrated aqueous solution of the lithium salt. Another such pair is water and ammonia.
Iron and iron alloys like steel and copper and copper alloys are typical construction materials used in absorption refrigeration systems. The corrosion of these metals can cause difficulties and in some cases, if unchecked, can shorten the life of the chiller interior components. Not only is metal loss of concern, but also the oxidation of ferrous metal produces gaseous hydrogen. If not purged, the hydrogen can interfere with the proper operation of the system. Corrosion is of particular concern in systems that use halogen salts of lithium, and regardless of the refrigerant/absorbent pair used in a particular system, metal corrosion rates increase as system temperatures increase.
It is well known in the prior art that the addition of a salt of chromium, such as lithium chromate, to the refrigerant/absorbent solution in an absorption refrigeration system, is effective in reducing metallic corrosion. The presence of the chromate compound promotes the formation of a protective layer of iron and chromium oxides on the surfaces of the system that are in contact with the absorbent. With a reduction in iron oxidation, there is also a corresponding reduction in the production of noncondensible hydrogen. There is some concern, however, about the health risks that chromium presents. At least one government authority, the U.S. Environmental Protection Agency, has identified chromium as a carcinogen, and has prohibited the presence of chromium compounds in systems that are open to the atmosphere. As of this writing, there is no restriction on the use of chromium compounds in closed systems but there may be such a restriction in the near future. Absorption refrigeration systems are, of course, closed systems, but a certain amount of working fluid from the system can become exposed to the atmosphere through the taking of samples, the manufacturing process and spills during handling and filing. And, at the end of the service life of a system, the system charge will necessarily require disposal of the working fluid, including the chromium compounds that it contains.
Currently available is a chromium-free aqueous solution of either a halogen salt of lithium or ammonia for use as a working fluid in a absorption refrigeration system. In addition to the lithium salt or ammonia, the solution also typically contains a compound containing a molybdate, a compound containing a borate and, in a preferred embodiment, a compound containing a silicate, so that ions of molybdenum, boron and silicon are present in the working fluid. A sufficient quantity of a compound containing a hydroxide is added to the solution to render it rather strongly basic. The added constituents act as effective corrosion inhibitors. The inhibiting performance of these fluids is superior to that of the prior lithium chromate inhibitors.
A working fluid is prepared by adding halogen or ammonium salts of molybdenum, boron and, in a preferred embodiment, silicon to the aqueous solution of the lithium salt or ammonia. A hydroxide of sodium, lithium, potassium or ammonium is also added to attain the desired alkalinity. The added compound are preferably added as aqueous solutions. This system is more fully described in U.S. Pat. No. 5,547,600 which is incorporated herein by reference.
Experience has indicated that new absorption chillers have a high inhibitor depletion rate which is predictable during its early hours of run time. A chiller operating with lower than recommended corrosion inhibitor concentration levels exposes the chiller interior to unacceptable corrosion rates and probable early life cycle failure. To provide ideal chiller corrosion protection, this rapid depletion of the initial inhibitor charge requires close observation of inhibitor concentration by solution sampling and timely replenishment by a skilled machine operator or service technician to maintain the desired inhibitor corrosion protection. This required personal attention is both costly and commonly leads to problems such a damaging corrosion rate due to mistakes in solution sampling and untimely inhibitor additions to the system.
It is therefore an object of the present invention to provide a corrosion replenishment system for an absorption refrigeration system which overcomes the problems of the prior art described above.
It is another object of the present invention to provide a corrosion inhibitor replenishment system which automatically provides corrosion protection for an absorption chiller.
It is a further object, of the present invention to provide a corrosion inhibitor system which is incorporated into the refrigerant flow during the operation of an absorption chiller.
It is yet another object of the present invention to provide at periodic time intervals an automatic time release process for supplying a corrosion inhibitor to an absorption chiller.
The present invention provides a system for unattended inhibitor periodic replenishment after selected numbers of hours of chiller operation. This is accomplished by using a coating to encapsulate the inhibitor which may be in pellet, granular, tablet, capsule or block form. The inhibitor coating will break down over specified periods of time when exposed to the absorption chiller refrigerant (water) thus allowing the inhibitor to be released into the refrigerant. This process is similar to that employed in the time release of certain medications and fertilizers which are released at predetermined time intervals, and spot preventing agents for dishwashers. At a selected location, the inhibitor would only be exposed to the refrigerant when the chiller is running. This would link the time encapsulated inhibitor is exposed to refrigerant to chiller run time. Optionally, the introduction of the inhibitor at the refrigerant spray header, would promote mixing as the refrigerant is circulated through the spray nozzles and refrigerant pump. The chiller electronic controls can be programmed to trigger a dilution cycle event at prescribed numbers of run time hours to allow the inhibitor rich refrigerant to mix with the LiBr solution in the absorber, and be circulated by the solution pump to the remainder of the chiller. This completes a cycle of unattended inhibitor replenishment on a timely basis to maintain the integrity of the corrosion protected surfaces of the complete chiller interior.